Could Fish Use Inertial Clues When on Migration?

Abstract

Plaice (Pleuronectes platessa) fitted with acoustic transponding, compass tags, released and tracked at sea with sector-scanning sonar, show a remarkably consistent heading when in midwater and at night under conditions when normal visual and tactile clues are absent. The results from the tracking studies are consistent with the hypothesis that the heading maintained in midwater may be selected by reference to topographical clues before the fish leaves the bottom. The suggestion that the fish could maintain a chosen heading in midwater by reference to intertial clues using the labyrinth as a detector is examined in the light of the known sensitivity of the semicircular canals to angular accelerations. A simple random walk model is used to calculate the extent to which a fish would be carried off course, when subjected to turning movements accompanying undetected angular accelerations of varying duration. It is shown that the spread of headings increases slowly with time for turning movements of high acceleration and short duration, but more rapidly with turning movements of low acceleration and long duration. If a fish were to restrict undetectable, and therefore uncorrectable, turns to a duration of less than 2 s by introducing movements of its own, the probability of the fish being within ±45° of its original heading after 3 h would be 0.46. It is suggested that the labyrinth could provide a limited facility for an inertial guidance system, which could be useful if the heading could be updated and corrected by reference to external clues at regular intervals. For plaice this could be provided by regular movements down into the bottom during the period spent in midwater. But this pattern of behaviour could not account for the consistent headings maintained by Atlantic salmon (Salmo salar) which keep close to the surface, and under these circumstances the question arises as to whether the fish is responding to inertial information provided by the orbital wave motion derived from the sea or swell.